Bat-derived influenza-like viruses H17N10 and H18N11

Influenza A Viruses Detected in Swine in Southern Germany after the H1N1 Pandemic in 2009

Infections with influenza A viruses (IAV) are highly prevalent in swine populations, and stable cocirculation of at least three lineages has been well documented in European swine - till 2009. However, since the emergence of the human pandemic pdmH1N1 virus in 2009, which has been (re)introduced into individual swine herds worldwide, the situation has been changing. These variations in the respective IAV pools within pig populations are of major interest, and the zoonotic potential of putative emerging viruses needs to be evaluated. As data on recent IAV in swine from southern Germany were relatively sparse, the purpose of this study was to determine the major IAV subtypes actually present in this region. To this aim, from 2010 to 2013, 1417 nasal swabs or lung tissue samples from pigs with respiratory disease were screened for IAV genomes. Overall, in 130 holdings IAV genomes were detected by real-time RT-PCR targeting the matrix protein gene. For further analyses, several PCR protocols were adapted to quickly subtype between H1, pdmH1, H3, N1 and N2 sequences. Taken together, cocirculation of the three stable European lineages of IAV was confirmed for Bavaria. H1N1 sequences were identified in 59, whereas H1N2 genomes were only diagnosed in 14, and H3N2 in 9 of the holdings analysed. However, pdmH1 in combination with N1 was detected in 2010, 2012 and 2013 confirming a presence, albeit in low prevalence, likewise pdmH1N2 reassortant viruses. Interestingly, individual cases of coinfections with more than one subtype were diagnosed. Partial genome sequences were determined and phylogenetic analyses performed. Clearly other than in the human population classically circulating IAV have not been displaced by pdmH1N1 in Bavarian swine. However, some interesting viruses were detected. Further surveillance of these viruses in the Bavarian pig population will be of major importance, to monitor future developments.

Vaccination against H5 avian influenza virus induces long-term humoral immune responses in flamingoes (Phoenicopterus spp.)

Avian influenza (AI) can represent a threat to endangered wild birds, as demonstrated with the H5N1 highly pathogenic AI (HPAI) outbreaks. Vaccination against AI using inactivated H5-vaccines has been shown to induce humoral immune response in zoo bird species. In this study, the long-term efficacy of H5-vaccination was evaluated in flamingoes from Barcelona Zoo. Specific H5-antibody titres were maintained at high levels (geometric mean titres ≥32) for over 7 years after vaccination, both against the H5N9 and H5N3 vaccine strains, as well as H5N3 and H5N1 reference strains. In addition the breadth of the immune response was also studied by testing antibody production against H1-, H3-, H4-, H7-, and H10-subtypes. It was observed that most flamingoes presented specific antibodies against H1 virus subtypes, but titres to the other HA-subtypes were rarely detected. We show that AI-vaccines can induce immunity lasting seven years in flamingoes, which suggests that vaccination can provide long term protection from HPAI outbreaks in zoo birds.

Phylogenetic analysis and pathogenicity of H3 subtype avian influenza viruses isolated from live poultry markets in China

H3 subtype influenza A virus is one of the main subtypes that threats both public and animal health. However, the evolution and pathogenicity of H3 avian influenza virus (AIV) circulating in domestic birds in China remain largely unclear. In this study, seven H3 AIVs (four H3N2 and three H3N8) were isolated from poultry in live poultry market (LPM) in China. Phylogenetic analyses of full genomes showed that all viruses were clustered into Eurasian lineage, except N8 genes of two H3N8 isolates fell into North American lineage. Intriguingly, the N8 gene of one H3N8 and PB2, PB1, NP and NS of two H3N2 isolates have close relationship with those of the highly pathogenic H5N8 viruses circulating in Korea and United States, suggesting that the H3-like AIV may contribute internal genes to the highly pathogenic H5N8 viruses. Phylogenetic tree of HA gene and antigenic cross-reactivity results indicated that two antigenically different H3 viruses are circulating in LPM in China. Most of the H3 viruses replicated in mice lung and nasal turbinate without prior adaptation, and the representative H3 viruses infected chickens without causing clinical signs. The reassortment of H3 subtype influenza viruses warrants continuous surveillance in LPM in China.

Human infection with influenza virus A(H10N8) from live poultry markets, China, 2014

Human infection with avian influenza virus A(H10N8) was initially reported in China in December 2013. We characterized H10N8 strains from a human patient and from poultry in live markets that infected persons had visited. Results of genome sequencing and virus characterization suggest that the virus strains that infected humans originated from these markets.

A Novel Potent and Highly Specific Inhibitor against Influenza Viral N1-N9 Neuraminidases: Insight into Neuraminidase-Inhibitor Interactions

People throughout the world continue to be at risk for death from influenza A virus, which is always creating a new variant. Here we present a new effective and specific anti-influenza viral neuraminidase (viNA) inhibitor, 9-cyclopropylcarbonylamino-4-guanidino-Neu5Ac2en (cPro-GUN). Like zanamivir, it is highly effective against N1-N9 avian and N1-N2 human viNAs, including H274Y oseltamivir-resistant N1 viNA, due to its C-6 portion still being anchored in the active site, different from the disruption of oseltamivir's C-6 anchoring by H274Y mutation. Unlike zanamivir, no sialidase inhibitory activity has been observed for cPro-GUN against huNeu1-huNeu4 enzymes. Broad efficacy of cPro-GUN against avian and human influenza viruses in cell cultures comparable to its sialidase inhibitory activities makes cPro-GUN ideal for further development for safe therapeutic or prophylactic use against both seasonal and pandemic influenza.

Masking of antigenic epitopes by antibodies shapes the humoral immune response to influenza

The immune responses to influenza, a virus that exhibits strain variation, show complex dynamics where prior immunity shapes the response to the subsequent infecting strains. Original antigenic sin (OAS) describes the observation that antibodies to the first encountered influenza strain, specifically antibodies to the epitopes on the head of influenza's main surface glycoprotein, haemagglutinin (HA), dominate following infection with new drifted strains. OAS suggests that responses to the original strain are preferentially boosted. Recent studies also show limited boosting of the antibodies to conserved epitopes on the stem of HA, which are attractive targets for a 'universal vaccine'. We develop multi-epitope models to explore how pre-existing immunity modulates the immune response to new strains following immunization. Our models suggest that the masking of antigenic epitopes by antibodies may play an important role in describing the complex dynamics of OAS and limited boosting of antibodies to the stem of HA. Analysis of recently published data confirms model predictions for how pre-existing antibodies to an epitope on HA decrease the magnitude of boosting of the antibody response to this epitope following immunization. We explore strategies for boosting of antibodies to conserved epitopes and generating broadly protective immunity to multiple strains.

Avian influenza viruses that cause highly virulent infections in humans exhibit distinct replicative properties in contrast to human H1N1 viruses

Avian influenza viruses present an emerging epidemiological concern as some strains of H5N1 avian influenza can cause severe infections in humans with lethality rates of up to 60%. These have been in circulation since 1997 and recently a novel H7N9-subtyped virus has been causing epizootics in China with lethality rates around 20%. To better understand the replication kinetics of these viruses, we combined several extensive viral kinetics experiments with mathematical modelling of in vitro infections in human A549 cells. We extracted fundamental replication parameters revealing that, while both the H5N1 and H7N9 viruses replicate faster and to higher titers than two low-pathogenicity H1N1 strains, they accomplish this via different mechanisms. While the H7N9 virions exhibit a faster rate of infection, the H5N1 virions are produced at a higher rate. Of the two H1N1 strains studied, the 2009 pandemic H1N1 strain exhibits the longest eclipse phase, possibly indicative of a less effective neuraminidase activity, but causes infection more rapidly than the seasonal strain. This explains, in part, the pandemic strain’s generally slower growth kinetics and permissiveness to accept mutations causing neuraminidase inhibitor resistance without significant loss in fitness. Our results highlight differential growth properties of H1N1, H5N1 and H7N9 influenza viruses.

Peramivir: A Novel Intravenous Neuraminidase Inhibitor for Treatment of Acute Influenza Infections

Peramivir is a novel cyclopentane neuraminidase inhibitor of influenza virus. It was approved by the Food and Drug Administration in December 2014 for treatment of acute uncomplicated influenza in patients 18 years and older. For several months prior to approval, the drug was made clinically available under Emergency Use authorization during the 2009 H1N1 influenza pandemic. Peramivir is highly effective against human influenza A and B isolates as well as emerging influenza virus strains with pandemic potential. Clinical trials demonstrated that the drug is well-tolerated in adult and pediatric populations. Adverse events are generally mild to moderate and similar in frequency to patients receiving placebo. Common side effects include gastrointestinal disorders and decreased neutrophil counts but are self-limiting. Peramivir is administered as a single-dose via the intravenous route providing a valuable therapeutic alternative for critically ill patients or those unable to tolerate other administration routes. Successful clinical trials and post-marketing data in pediatric populations in Japan support the safety and efficacy of peramivir in this population where administration of other antivirals might not be feasible.

The Hemagglutinin of Bat-Associated Influenza Viruses Is Activated by TMPRSS2 for pH-Dependent Entry into Bat but Not Human Cells

New World bats have recently been discovered to harbor influenza A virus (FLUAV)-related viruses, termed bat-associated influenza A-like viruses (batFLUAV). The internal proteins of batFLUAV are functional in mammalian cells. In contrast, no biological functionality could be demonstrated for the surface proteins, hemagglutinin (HA)-like (HAL) and neuraminidase (NA)-like (NAL), and these proteins need to be replaced by their human counterparts to allow spread of batFLUAV in human cells. Here, we employed rhabdoviral vectors to study the role of HAL and NAL in viral entry. Vectors pseudotyped with batFLUAV-HAL and -NAL were able to enter bat cells but not cells from other mammalian species. Host cell entry was mediated by HAL and was dependent on prior proteolytic activation of HAL and endosomal low pH. In contrast, sialic acids were dispensable for HAL-driven entry. Finally, the type II transmembrane serine protease TMPRSS2 was able to activate HAL for cell entry indicating that batFLUAV can utilize human proteases for HAL activation. Collectively, these results identify viral and cellular factors governing host cell entry driven by batFLUAV surface proteins. They suggest that the absence of a functional receptor precludes entry of batFLUAV into human cells while other prerequisites for entry, HAL activation and protonation, are met in target cells of human origin.

Platform technology to generate broadly cross-reactive antibodies to α-helical epitopes in hemagglutinin proteins from influenza A viruses

We have utilized a de novo designed two‐stranded α‐helical coiled‐coil template to display conserved α‐helical epitopes from the stem region of hemagglutinin (HA) glycoproteins of influenza A. The immunogens have all the surface‐exposed residues of the native α‐helix in the native HA protein of interest displayed on the surface of the two‐stranded α‐helical coiled‐coil template. This template when used as an immunogen elicits polyclonal antibodies which bind to the α‐helix in the native protein. We investigated the highly conserved sequence region 421–476 of HA by inserting 21 or 28 residue sequences from this region into our template. The cross‐reactivity of the resulting rabbit polyclonal antibodies prepared to these immunogens was determined using a series of HA proteins from H1N1, H2N2, H3N2, H5N1, H7N7, and H7N9 virus strains which are representative of Group 1 and Group 2 virus subtypes of influenza A. Antibodies from region 449–476 were Group 1 specific. Antibodies to region 421–448 showed the greatest degree of cross‐reactivity to Group 1 and Group 2 and suggested that this region has a great potential as a “universal” synthetic peptide vaccine for influenza A. © 2016 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 106: 144–159, 2016.

Stalking influenza by vaccination with pre-fusion headless HA mini-stem

Inaccuracies in prediction of circulating viral strain genotypes and the possibility of novel reassortants causing a pandemic outbreak necessitate the development of an anti-influenza vaccine with increased breadth of protection and potential for rapid production and deployment. The hemagglutinin (HA) stem is a promising target for universal influenza vaccine as stem-specific antibodies have the potential to be broadly cross-reactive towards different HA subtypes. Here, we report the design of a bacterially expressed polypeptide that mimics a H5 HA stem by protein minimization to focus the antibody response towards the HA stem. The HA mini-stem folds as a trimer mimicking the HA prefusion conformation. It is resistant to thermal/chemical stress, and it binds to conformation-specific, HA stem-directed broadly neutralizing antibodies with high affinity. Mice vaccinated with the group 1 HA mini-stems are protected from morbidity and mortality against lethal challenge by both group 1 (H5 and H1) and group 2 (H3) influenza viruses, the first report of cross-group protection. Passive transfer of immune serum demonstrates the protection is mediated by stem-specific antibodies. Furthermore, antibodies indudced by these HA stems have broad HA reactivity, yet they do not have antibody-dependent enhancement activity.

Host-Specific and Segment-Specific Evolutionary Dynamics of Avian and Human Influenza A Viruses: A Systematic Review

Understanding the evolutionary dynamics of influenza viruses is essential to control both avian and human influenza. Here, we analyze host-specific and segment-specific Tajima's D trends of influenza A virus through a systematic review using viral sequences registered in the National Center for Biotechnology Information. To avoid bias from viral population subdivision, viral sequences were stratified according to their sampling locations and sampling years. As a result, we obtained a total of 580 datasets each of which consists of nucleotide sequences of influenza A viruses isolated from a single population of hosts at a single sampling site within a single year. By analyzing nucleotide sequences in the datasets, we found that Tajima's D values of viral sequences were different depending on hosts and gene segments. Tajima's D values of viruses isolated from chicken and human samples showed negative, suggesting purifying selection or a rapid population growth of the viruses. The negative Tajima's D values in rapidly growing viral population were also observed in computer simulations. Tajima's D values of PB2, PB1, PA, NP, and M genes of the viruses circulating in wild mallards were close to zero, suggesting that these genes have undergone neutral selection in constant-sized population. On the other hand, Tajima's D values of HA and NA genes of these viruses were positive, indicating HA and NA have undergone balancing selection in wild mallards. Taken together, these results indicated the existence of unknown factors that maintain viral subtypes in wild mallards.

Molecular Characterization of Subtype H11N9 Avian Influenza Virus Isolated from Shorebirds in Brazil

Migratory aquatic birds play an important role in the maintenance and spread of avian influenza viruses (AIV). Many species of aquatic migratory birds tend to use similar migration routes, also known as flyways, which serve as important circuits for the dissemination of AIV. In recent years there has been extensive surveillance of the virus in aquatic birds in the Northern Hemisphere; however in contrast only a few studies have been attempted to detect AIV in wild birds in South America. There are major flyways connecting South America to Central and North America, whereas avian migration routes between South America and the remaining continents are uncommon. As a result, it has been hypothesized that South American AIV strains would be most closely related to the strains from North America than to those from other regions in the world. We characterized the full genome of three AIV subtype H11N9 isolates obtained from ruddy turnstones (Arenaria interpres) on the Amazon coast of Brazil. For all gene segments, all three strains consistently clustered together within evolutionary lineages of AIV that had been previously described from aquatic birds in North America. In particular, the H11N9 isolates were remarkably closely related to AIV strains from shorebirds sampled at the Delaware Bay region, on the Northeastern coast of the USA, more than 5000 km away from where the isolates were retrieved. Additionally, there was also evidence of genetic similarity to AIV strains from ducks and teals from interior USA and Canada. These findings corroborate that migratory flyways of aquatic birds play an important role in determining the genetic structure of AIV in the Western hemisphere, with a strong epidemiological connectivity between North and South America.

DNA-based influenza vaccines as immunoprophylactic agents toward universality

Influenza is an illness of global public health concern. Influenza viruses have been responsible for several pandemics affecting humans. Current influenza vaccines have proved satisfactory safety; however, they have limitations and do not provide protection against unexpected emerging influenza virus strains. Therefore, there is an urgent need for alternative approaches to conventional influenza vaccines. The development of universal influenza vaccines will help alleviate the severity of influenza pandemics. Influenza DNA vaccines have been the subject of many studies over the past decades due to their ability to induce broad-based protective immune responses in various animal models. The present review highlights the recent advances in influenza DNA vaccine research and its potential as an affordable universal influenza vaccine.

High Antibody-Dependent Cellular Cytotoxicity Antibody Titers to H5N1 and H7N9 Avian Influenza A Viruses in Healthy US Adults and Older Children

Human influenza is a highly contagious acute respiratory illness that is responsible for significant morbidity and excess mortality worldwide. In addition to neutralizing antibodies, there are antibodies that bind to influenza virus-infected cells and mediate lysis of the infected cells by natural killer (NK) cells (antibody-dependent cellular cytotoxicity [ADCC]) or complement (complement-dependent lysis [CDL]). We analyzed sera obtained from 16 healthy adults (18-63 years of age), 52 children (2-17 years of age), and 10 infants (0.75-1 year of age) in the United States, who were unlikely to have been exposed to the avian H7N9 subtype of influenza A virus, by ADCC and CDL assays. As expected, none of these sera had detectable levels of hemagglutination-inhibiting antibodies against the H7N9 virus, but we unexpectedly found high titers of ADCC antibodies to the H7N9 subtype virus in all sera from adults and children aged ≥8 years.

Immunogenicity and protective efficacy of the norovirus P particle-M2e chimeric vaccine in chickens

The ectodomain of the influenza matrix protein 2 (M2e) is highly conserved across strains and has been shown to be a promising candidate for universal influenza vaccine in the mouse model. In this study, we tested immune response and protective efficacy of a chimeric norovirus P particle containing the avian M2e protein against challenges with three avian influenza (AI) viruses (H5N2, H6N2, H7N2) in chickens. Two-week-old specific pathogen free chickens were vaccinated 3 times with an M2e-P particle (M2e-PP) vaccine via the subcutaneous (SQ) route with oil adjuvant, and transmucosal routes (intranasal, IN; eye drop, ED; microspray, MS) without adjuvant. M2e-PP vaccination via the SQ route induced significant IgG antibody responses which were increased by each booster vaccination. In groups vaccinated via IN, ED or MS, neither IgG nor IgA responses were detected from sera or nasal washes of immunized birds. The M2e-PP vaccination via the SQ route significantly reduced the virus shedding in the trachea and the cloaca for all three challenge viruses. Despite the absence of detectable IgG and IgA responses in birds vaccinated with the M2e-PP via intranasal routes, a similar level of reduction in virus shedding was observed in the IN group compared to the SQ group. Our results supports that the universal vaccine approach using M2e-based vaccine can provide cross-protection against challenge viruses among different HA subtypes although the efficacy of the vaccine should be enhanced further to be practical. Better understanding of the protective immune mechanism will be critical for the development of an M2e-based vaccine in chickens.

A potent broad-spectrum protective human monoclonal antibody crosslinking two haemagglutinin monomers of influenza A virus

Effective annual influenza vaccination requires frequent changes in vaccine composition due to both antigenic shift for different subtype hemagglutinins (HAs) and antigenic drift in a particular HA. Here we present a broadly neutralizing human monoclonal antibody with an unusual binding modality. The antibody, designated CT149, was isolated from convalescent patients infected with pandemic H1N1 in 2009. CT149 is found to neutralize all tested group 2 and some group 1 influenza A viruses by inhibiting low pH-induced, HA-mediated membrane fusion. It promotes killing of infected cells by Fc-mediated antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity. X-ray crystallographic data reveal that CT149 binds primarily to the fusion domain in HA2, and the light chain is also largely involved in binding. The epitope recognized by this antibody comprises amino-acid residues from two adjacent protomers of HA. This binding characteristic of CT149 will provide more information to support the design of more potent influenza vaccines.

Systemic immune responses to an inactivated, whole H9N2 avian influenza virus vaccine using class B CpG oligonucleotides in chickens

Commercial vaccines against avian influenza viruses (AIV) in chickens consist mainly of inactivated AIV, requiring parenteral administration and co-delivery of an adjuvant. Limitations in T helper 1 or T helper 2 biased responses generated by these vaccines emphasize the need for alternative, more efficacious adjuvants. The Toll-like receptor (TLR) 21 ligand, CpG oligodeoxynucleotides (ODN), has been established as immunomodulatory in chickens. Therefore, the objective of this study was to investigate the adjuvant potential of high (20μg) and low (2μg) doses of CpG ODN 2007 (CpG 2007) and CpG ODN 1826 (CpG 1826) when administered to chickens with a formalin-inactivated H9N2 AIV. Antibody responses in sera were evaluated in 90 specific pathogen free (SPF) chickens after intramuscular administration of vaccine formulations at 7 and 21 days post-hatch. Antibody responses were assessed based on haemagglutination inhibition (HI) and virus neutralization (VN) assays; virus-specific IgM and IgY antibody responses were evaluated by ELISA. The results suggest that the vaccine formulation containing low dose CpG 2007 was significantly more effective at generating neutralizing (both HI and VN) responses than formulations with high or low doses of CpG 1826 or high dose CpG 2007. Neutralizing responses elicited by low dose CpG 2007 significantly exceeded those generated by a squalene-based adjuvanted vaccine formulation during peak responses. A significantly higher IgM response was elicited by the formulation containing low dose CpG 2007 compared to high and low doses of 1826. Although the low dose of CpG 2007 elicited a higher IgY response than CpG 1826, the difference was not statistically significant. In conclusion, 2μg of CpG 2007 is potentially promising as a vaccine adjuvant when delivered intramuscularly with inactivated H9N2 virus to chickens. Future studies may be directed at determining the mucosal antibody responses to the same vaccine formulations.

Cross-Reactive Neuraminidase-Inhibiting Antibodies Elicited by Immunization with Recombinant Neuraminidase Proteins of H5N1 and Pandemic H1N1 Influenza A Viruses

Neuraminidase (NA), an influenza virus envelope glycoprotein, removes sialic acid from receptors for virus release from infected cells. For this study, we used a baculovirus-insect cell expression system to construct and purify recombinant NA (rNA) proteins of H5N1 (A/Vietnam/1203/2004) and pandemic H1N1 (pH1N1) (A/Texas/05/2009) influenza viruses. BALB/c mice immunized with these proteins had high titers of NA-specific IgG and NA-inhibiting (NI) antibodies against H5N1, pH1N1, H3N2, and H7N9 viruses. H5N1 rNA immunization resulted in higher quantities of NA-specific antibody-secreting B cells against H5N1 and heterologous pH1N1 viruses in the spleen. H5N1 rNA and pH1N1 rNA immunizations both provided complete protection against homologous virus challenges, with H5N1 rNA immunization providing better protection against pH1N1 virus challenges. Cross-reactive NI antibodies were further dissected via pH1N1 rNA protein immunizations with I149V (NA with a change of Ile to Val at position 149), N344Y, and I365T/S366N NA mutations. The I365T/S366N mutation of pH1N1 rNA enhanced cross-reactive NI antibodies against H5N1, H3N2, and H7N9 viruses. It is our hope that these findings provide useful information for the development of an NA-based universal influenza vaccine.

IMPORTANCE

Neuraminidase (NA) is an influenza virus enzymatic protein that cleaves sialic acid linkages on infected cell surfaces, thus facilitating viral release and contributing to viral transmission and mucus infection. In currently available inactivated or live, attenuated influenza vaccines based on the antigenic content of hemagglutinin proteins, vaccine efficacy can be contributed partly through NA-elicited immune responses. We investigated the NA immunity of different recombinant NA (rNA) proteins associated with pH1N1 and H5N1 viruses. Our results indicate that H5N1 rNA immunization induced more potent cross-protective immunity than pH1N1 rNA immunization, and three mutated residues, I149V, I365T, and S366N, near the NA enzyme active site(s) are linked to enhanced cross-reactive NA-inhibiting antibodies against heterologous and heterosubtypic influenza A viruses. These findings provide useful information for the development of an NA-based universal influenza vaccine.

Adaptation of avian influenza A (H6N1) virus from avian to human receptor-binding preference

The receptor-binding specificity of influenza A viruses is a major determinant for the host tropism of the virus, which enables interspecies transmission. In 2013, the first human case of infection with avian influenza A (H6N1) virus was reported in Taiwan. To gather evidence concerning the epidemic potential of H6 subtype viruses, we performed comprehensive analysis of receptor-binding properties of Taiwan-isolated H6 HAs from 1972 to 2013. We propose that the receptor-binding properties of Taiwan-isolated H6 HAs have undergone three major stages: initially avian receptor-binding preference, secondarily obtaining human receptor-binding capacity, and recently human receptor-binding preference, which has been confirmed by receptor-binding assessment of three representative virus isolates. Mutagenesis work revealed that E190V and G228S substitutions are important to acquire the human receptor-binding capacity, and the P186L substitution could reduce the binding to avian receptor. Further structural analysis revealed how the P186L substitution in the receptor-binding site of HA determines the receptor-binding preference change. We conclude that the human-infecting H6N1 evolved into a human receptor preference.

A Single Dose of an Avian H3N8 Influenza Virus Vaccine Is Highly Immunogenic and Efficacious against a Recently Emerged Seal Influenza Virus in Mice and Ferrets

H3N8 influenza viruses are a commonly found subtype in wild birds, usually causing mild or no disease in infected birds. However, they have crossed the species barrier and have been associated with outbreaks in dogs, pigs, donkeys, and seals and therefore pose a threat to humans. A live attenuated, cold-adapted (ca) H3N8 vaccine virus was generated by reverse genetics using the wild-type (wt) hemagglutinin (HA) and neuraminidase (NA) genes from the A/blue-winged teal/Texas/Sg-00079/2007 (H3N8) (tl/TX/079/07) wt virus and the six internal protein gene segments from the ca influenza A virus vaccine donor strain, A/Ann Arbor/6/60 ca (H2N2), the backbone of the licensed seasonal live attenuated influenza vaccine. One dose of the tl/TX/079/07 ca vaccine induced a robust neutralizing antibody response against the homologous (tl/TX/079/07) and two heterologous influenza viruses, including the recently emerged A/harbor seal/New Hampshire/179629/2011 (H3N8) and A/northern pintail/Alaska/44228-129/2006 (H3N8) viruses, and conferred robust protection against the homologous and heterologous influenza viruses. We also analyzed human sera against the tl/TX/079/07 H3N8 avian influenza virus and observed low but detectable antibody reactivity in elderly subjects, suggesting that older H3N2 influenza viruses confer some cross-reactive antibody. The latter observation was confirmed in a ferret study. The safety, immunogenicity, and efficacy of the tl/TX/079/07 ca vaccine in mice and ferrets support further evaluation of this vaccine in humans for use in the event of transmission of an H3N8 avian influenza virus to humans. The human and ferret serology data suggest that a single dose of the vaccine may be sufficient in older subjects.

IMPORTANCE

Although natural infection of humans with an avian H3N8 influenza virus has not yet been reported, this influenza virus subtype has already crossed the species barrier and productively infected mammals. Pandemic preparedness is an important public health priority. Therefore, we generated a live attenuated avian H3N8 vaccine candidate and demonstrated that a single dose of the vaccine was highly immunogenic and protected mice and ferrets against homologous and heterologous H3N8 avian viruses.

Distribution of sialic acid receptors and experimental infections with different subtypes of influenza A viruses in Qinghai-Tibet plateau wild pika

Background

The plateau pika (Ochotona curzoniae) is a small rabbit-like mammal that lives at high altitudes in the Qinghai-Tibet plateau and is in close contact with birds. Following the outbreak of highly pathogenic avian influenza (HPAI) H5N1 during 2005 in the migratory birds of Qinghai Lake, two clades of H5N1 have been found in pikas. However, the influenza virus receptor distribution in different tissues of this animal and its susceptibility to influenza A viruses have remained unclear.

Methods

The sialic acid receptor distribution tropism in pika was investigated using fluorescent Sambucus nigra and biotinylated Maackia amurensis I and II. Furthermore, the replication of three influenza A viruses H1N1, H3N2, and H5N1 in this animal was examined by immunohistochemistry and RT-PCR. Morphological and histopathological changes caused by infection were also analyzed with hematoxylin and eosin (H & E) staining.

Results

Human influenza virus-recognizing SAα2,6Gal receptors are widely expressed in the lung, kidney, liver, spleen, duodenum, ileum, rectum, and heart, whereas avian influenza virus-recognizing SAα2,3Gal receptors are strongly expressed in the trachea and lung of pika. M1 could be detected in the lungs of pikas infected with H1N1, H3N2, and H5N1 by either immunostaining or RT-PCR, and in the brain of H5N1-infected pikas. Additionally, three subtypes of influenza A viruses were able to infect pika and caused varying degrees of pneumonia with epithelial desquamation and alveolar inflammatory cell infiltration. Slight pathological changes were observed in H1N1-infected lungs. A few small bronchi and terminal bronchioles were infiltrated by lymphocytic cells in H3N2-infected lungs. In contrast, serious lung damage, such as alveolar capillary hyperemia, edema, alveolar collapse, and lymphocytic infiltrations was observed in H5N1-infected group. Furthermore, neural system changes were present in the brains of H5N1-infected pikas.

Conclusions

SAα2,6Gal receptors are extensively present in many of the tissues and organs in wild plateau pika, whereas SA2,3Gal-linked receptors are dominant on the tracheal epithelial cells. H1N1, H3N2, and H5N1 were able to infect pika and caused different degrees of pathogenic changes in the lungs. Altogether, these results suggest that wild pika has the potential to be a host for different subtypes of influenza A viruses.

Electronic supplementary material

The online version of this article (doi:10.1186/s12985-015-0290-8) contains supplementary material, which is available to authorized users.

Rapid emergence of a PB2-E627K substitution confers a virulent phenotype to an H9N2 avian influenza virus during adoption in mice

The worldwide circulation of H9N2 avian influenza virus in poultry, the greater than 2.3 % positive rate for anti-H9 antibodies in poultry-exposed workers, and several reports of human infection indicate that H9N2 virus is a potential threat to human health. Here, we found three mutations that conferred high virulence to H9N2 virus in mice after four passages. The PB2-E627K substitution rapidly appeared at the second passage and played a decisive role in virulence. Polymerase complexes possessing PB2-E627K displayed 16.1-fold higher viral polymerase activity when compared to the wild-type virus, which may account for enhanced virulence of this virus. The other two substitutions (HA-N313D and HA-N496S) enhanced binding to both α2,3-linked and α2,6-linked sialic acid receptors; however, the HA-N313D and N496S substitutions alone decreased the virulence of mouse-adapted virus. Furthermore, this mouse-adapted virus was still not transmissible among guinea pigs by direct contact (0/3 pairs). Our findings show that adaption in mice enhanced the viral polymerase activity and receptor-binding ability, which resulted in a virulent phenotype in mice but not a transmissible phenotype in guinea pigs, indicating that host factors play an important role in adaptive evolution of influenza in new hosts.

The role of viral, host, and secondary bacterial factors in influenza pathogenesis

Influenza A virus infections in humans generally cause self-limited infections, but can result in severe disease, secondary bacterial pneumonias, and death. Influenza viruses can replicate in epithelial cells throughout the respiratory tree and can cause tracheitis, bronchitis, bronchiolitis, diffuse alveolar damage with pulmonary edema and hemorrhage, and interstitial and airspace inflammation. The mechanisms by which influenza infections result in enhanced disease, including development of pneumonia and acute respiratory distress, are multifactorial, involving host, viral, and bacterial factors. Host factors that enhance risk of severe influenza disease include underlying comorbidities, such as cardiac and respiratory disease, immunosuppression, and pregnancy. Viral parameters enhancing disease risk include polymerase mutations associated with host switch and adaptation, viral proteins that modulate immune and antiviral responses, and virulence factors that increase disease severity, which can be especially prominent in pandemic viruses and some zoonotic influenza viruses causing human infections. Influenza viral infections result in damage to the respiratory epithelium that facilitates secondary infection with common bacterial pneumopathogens and can lead to secondary bacterial pneumonias that greatly contribute to respiratory distress, enhanced morbidity, and death. Understanding the molecular mechanisms by which influenza and secondary bacterial infections, coupled with the role of host risk factors, contribute to enhanced morbidity and mortality is essential to develop better therapeutic strategies to treat severe influenza.

Alterations in hemagglutinin receptor-binding specificity accompany the emergence of highly pathogenic avian influenza viruses

Highly pathogenic avian influenza viruses (HPAIVs) of hemagglutinin H5 and H7 subtypes emerge after introduction of low-pathogenic avian influenza viruses (LPAIVs) from wild birds into poultry flocks, followed by subsequent circulation and evolution. The acquisition of multiple basic amino acids at the endoproteolytical cleavage site of the hemagglutinin (HA) is a molecular indicator for high pathogenicity, at least for infections of gallinaceous poultry. Apart from the well-studied significance of the multibasic HA cleavage site, there is only limited knowledge on other alterations in the HA and neuraminidase (NA) molecules associated with changes in tropism during the emergence of HPAIVs from LPAIVs. We hypothesized that changes in tropism may require alterations of the sialyloligosaccharide specificities of HA and NA. To test this hypothesis, we compared a number of LPAIVs and HPAIVs for their HA-mediated binding and NA-mediated desialylation of a set of synthetic receptor analogs, namely, α2-3-sialylated oligosaccharides. NA substrate specificity correlated with structural groups of NAs and did not correlate with pathogenic potential of the virus. In contrast, all HPAIVs differed from LPAIVs by a higher HA receptor-binding affinity toward the trisaccharides Neu5Acα2-3Galβ1-4GlcNAcβ (3'SLN) and Neu5Acα2-3Galβ1-3GlcNAcβ (SiaLe(c)) and by the ability to discriminate between the nonfucosylated and fucosylated sialyloligosaccharides 3'SLN and Neu5Acα2-3Galβ1-4(Fucα1-3)GlcNAcβ (SiaLe(x)), respectively. These results suggest that alteration of the receptor-binding specificity accompanies emergence of the HPAIVs from their low-pathogenic precursors.

IMPORTANCE

Here, we have found for the first time correlations of receptor-binding properties of the HA with a highly pathogenic phenotype of poultry viruses. Our study suggests that enhanced receptor-binding affinity of HPAIVs for a typical "poultry-like" receptor, 3'SLN, is provided by substitutions in the receptor-binding site of HA which appeared in HA of LPAIVs in the course of transmission of LPAIVs from wild waterfowl into poultry flocks, with subsequent adaptation in poultry. The identification of LPAIVs with receptor characteristics of HPAIVs argues that the sialic acid-binding specificity of the HA may be used as a novel phenotypic marker of HPAIVs.

Functional properties and genetic relatedness of the fusion and hemagglutinin-neuraminidase proteins of a mumps virus-like bat virus

A bat virus with high phylogenetic relatedness to human mumps virus (MuV) was identified recently at the nucleic acid level. We analyzed the functional activities of the hemagglutinin-neuraminidase (HN) and the fusion (F) proteins of the bat virus (batMuV) and compared them to the respective proteins of a human isolate. Transfected cells expressing the F and HN proteins of batMuV were recognized by antibodies directed against these proteins of human MuV, indicating that both viruses are serologically related. Fusion, hemadsorption, and neuraminidase activities were demonstrated for batMuV, and either bat-derived protein could substitute for its human MuV counterpart in inducing syncytium formation when coexpressed in different mammalian cell lines, including chiropteran cells. Cells expressing batMuV glycoproteins were shown to have lower neuraminidase activity. The syncytia were smaller, and they were present in lower numbers than those observed after coexpression of the corresponding glycoproteins of a clinical isolate of MuV (hMuV). The phenotypic differences in the neuraminidase and fusion activity between the glycoproteins of batMuV and hMuV are explained by differences in the expression level of the HN and F proteins of the two viruses. In the case of the F protein, analysis of chimeric proteins revealed that the signal peptide of the bat MuV fusion protein is responsible for the lower surface expression. These results indicate that the surface glycoproteins of batMuV are serologically and functionally related to those of hMuV, raising the possibility of bats as a reservoir for interspecies transmission.

IMPORTANCE

The recently described MuV-like bat virus is unique among other recently identified human-like bat-associated viruses because of its high sequence homology (approximately 90% in most genes) to its human counterpart. Although it is not known if humans can be infected by batMuV, the antigenic relatedness between the bat and human forms of the virus suggests that humans carrying neutralizing antibodies against MuV are protected from infection by batMuV. The close functional relationship between MuV and batMuV is demonstrated by cooperation of the respective HN and F proteins to induce syncytium formation in heterologous expression studies. An interesting feature of the glycoproteins of batMuV is the downregulation of the fusion activity by the signal peptide of F, which has not been reported for other paramyxoviruses. These results are important contributions for risk assessment and for a better understanding of the replication strategy of batMuV.

Structure and receptor binding preferences of recombinant hemagglutinins from avian and human H6 and H10 influenza A virus subtypes

During 2013, three new avian influenza A virus subtypes, A(H7N9), A(H6N1), and A(H10N8), resulted in human infections. While the A(H7N9) virus resulted in a significant epidemic in China across 19 provinces and municipalities, both A(H6N1) and A(H10N8) viruses resulted in only a few human infections. This study focuses on the major surface glycoprotein hemagglutinins from both of these novel human viruses. The detailed structural and glycan microarray analyses presented here highlight the idea that both A(H6N1) and A(H10N8) virus hemagglutinins retain a strong avian receptor binding preference and thus currently pose a low risk for sustained human infections.

IMPORTANCE

Human infections with zoonotic influenza virus subtypes continue to be a great public health concern. We report detailed structural analysis and glycan microarray data for recombinant hemagglutinins from A(H6N1) and A(H10N8) viruses, isolated from human infections in 2013, and compare them with hemagglutinins of avian origin. This is the first structural report of an H6 hemagglutinin, and our results should further the understanding of these viruses and provide useful information to aid in the continuous surveillance of these zoonotic influenza viruses.

Avian Influenza Virus with Hemagglutinin-Neuraminidase Combination H3N6, Isolated from a Domestic Pigeon in Guangxi, Southern China

The H3 subtype of avian influenza virus can provide genes for human influenza virus through gene reassortment, which has raised great concerns about its potential threat to human health. An H3N6 subtype of avian influenza virus was isolated from Guangxi Province, China, in 2009. All eight gene segments of the strain were sequenced. The sequence analysis indicated that this H3N6 virus was a nature reassortant virus. The genome sequences now can be used to understand the epidemiological and molecular characteristics of the H3N6 influenza virus in southern China.

Adenovirus-based vaccines against avian-origin H5N1 influenza viruses

Since 1997, human infection with avian H5N1, having about 60% mortality, has posed a threat to public health. In this review, we describe the epidemiology of H5N1 transmission, advantages and disadvantages of different influenza vaccine types, and characteristics of adenovirus, finally summarizing advances in adenovirus-based H5N1 systemic and mucosal vaccines.

Efficient generation of recombinant influenza A viruses employing a new approach to overcome the genetic instability of HA segments

Influenza A viruses (IAVs) are the most relevant and continual source of severe infectious respiratory complications in humans and different animal species, especially poultry. Therefore, an efficient vaccination that elicits protective and neutralizing antibodies against the viral hemagglutinin (HA) and neuraminidase (NA) is an important strategy to counter annual epidemics or occasional pandemics. With the help of plasmid-based reverse genetics technology, it is possible that IAV vaccine strains (IVVS) are rapidly generated. However, the genetic instability of some cloned HA-cDNAs after transformation into competent bacteria represents a major obstacle. Herein, we report efficient cloning strategies of different genetically volatile HA segments (H5- and H9-subtypes) employing either a newly constructed vector for reverse genetics (pMKPccdB) or by the use of the Escherichia coli strain HB101. Both approaches represent improved and generalizable strategies to establish functional reverse genetics systems preventing genetic changes to the cloned (HA) segments of IAV facilitating more efficient rescue of recombinant IAV for basic research and vaccine development.

Influenza virus reservoirs and intermediate hosts: dogs, horses, and new possibilities for influenza virus exposure of humans

Influenza A virus (IAV) infections in hosts outside the main aquatic bird reservoirs occur periodically. Although most such cross-species transmission events result in limited onward transmission in the new host, sustained influenza outbreaks have occurred in poultry and in a number of mammalian species, including humans, pigs, horses, seals, and mink. Recently, two distinct strains of IAV have emerged in domestic dogs, with each circulating widely for several years. Here, we briefly outline what is known about the role of intermediate hosts in influenza emergence, summarize our knowledge of the new canine influenza viruses (CIVs) and how they provide key new information on the process of host adaptation, and assess the risk these viruses pose to human populations.

[Fowl plague and avian influenza A viruses of poultry and birds. Diagnosis, control measures and practical experiences]

The causes of the notifiable fowl plague are high and low pathogenic avian influenza A viruses of the haemagglutinin subtypes H5 and H7 but also other haemagglutinin subtypes If the intravenous pathogenicity index is greater than 1.2. The German fowl plague order (Geflügelpest-Verordnung) differentiates between highly pathogenic influenza A viruses of the subtypes H5 and H7, if multiple basic amino acids at the cleavage site of the haemagglutinin molecules are detected by virus isolation, antigen or genome determination and low pathogenic avian influenza A viruses of the subtypes H5 and H7 if either the intravenous pathogenicity index is lower than 1.2 or no basic amino acids are present at the cleavage site of the haemagglutinin molecule. Aspects of diagnosis, control including culling, therapy and vaccination are reviewed. The currently available means and their limitations of a therapy of fowl plague by oral administration of neuraminidase inhibitors (e. g. oseltamivir) are described. Following granted permission, individually marked valuable zoo and pet birds may be vaccinated using licensed inactivated vaccines. Vector vaccines have not been used in Germany so far. Avian influenza A viruses of other haemagglutinin subtypes (H1-H4, H6, H8-H18) may also cause infections and severe disease. These subtypes are not subject to governmental interventions and disease can be prevented by timely use of inactivated vaccines.

Enabling the 'host jump': structural determinants of receptor-binding specificity in influenza A viruses

The recent emergence of the H7N9 avian influenza A virus and its ability to infect humans emphasize the epidemic and pandemic potential of these viruses. Interspecies transmission is the result of many factors, which ultimately lead to a change in the host tropism of the virus. One of the key factors involved is a shift in the receptor-binding specificity of the virus, which is mostly determined by mutations in the viral haemagglutinin (HA). In this Review, we discuss recent crystallographic studies that provide molecular insights into HA-host receptor interactions that have enabled several influenza A virus subtypes to 'jump' from avian to human hosts.

Assessment of the internal genes of influenza A (H7N9) virus contributing to high pathogenicity in mice

The recently identified H7N9 influenza A virus has caused severe economic losses and worldwide public concern. Genetic analysis indicates that its six internal genes all originated from H9N2 viruses. However, the H7N9 virus is more highly pathogenic in humans than H9N2, which suggests that the internal genes of H7N9 have mutated. To analyze which H7N9 virus internal genes contribute to its high pathogenicity, a series of reassortants was generated by reverse genetics, with each virus containing a single internal gene of the typical A/Anhui/1/2013 (H7N9) (AH-H7N9) virus in the genetic background of the A/chicken/Shandong/lx1023/2007 (H9N2) virus. The replication ability, polymerase activity, and pathogenicity of these viruses were then evaluated in vitro and in vivo. These recombinants displayed high genetic compatibility, and the H7N9-derived PB2, M, and NP genes were identified as the virulence genes for the reassortants in mice. Further investigation confirmed that the PB2 K627 residue is critical for the high pathogenicity of the H7N9 virus and the reassortant containing the H7N9-derived PB2 segment (H9N2-AH/PB2). Notably, the H7N9-derived PB2 gene displayed greater compatibility with the H9N2 genome than that of H7N9, endowing the H9N2-AH/PB2 reassortant with greater viability and virulence than the parental H7N9 virus. In addition, the H7N9 virus, with the exception of the H9N2 reassortants, could effectively replicate in human A549 cells. Our results indicate that PB2, M, and NP are the key virulence genes, together with the surface hemagglutinin (HA) and neuraminidase (NA) proteins, contributing to the high infectivity of the H7N9 virus in humans.

IMPORTANCE

To date, the novel H7N9 influenza A virus has caused 437 human infections, with approximately 30% mortality. Previous work has primarily focused on the two viral surface proteins, HA and NA, but the contribution of the six internal genes to the high pathogenicity of H7N9 has not been systematically studied. Here, the H9N2 virus was used as a genetic backbone to evaluate the virulence genes of H7N9 virus in vitro and in vivo. Our data indicate that the PB2, M, and NP genes play important roles in viral infection in mice and, together with HA and NA, contribute to the high infectivity of the H7N9 virus in humans.

Characterization of uncultivable bat influenza virus using a replicative synthetic virus

Bats harbor many viruses, which are periodically transmitted to humans resulting in outbreaks of disease (e.g., Ebola, SARS-CoV). Recently, influenza virus-like sequences were identified in bats; however, the viruses could not be cultured. This discovery aroused great interest in understanding the evolutionary history and pandemic potential of bat-influenza. Using synthetic genomics, we were unable to rescue the wild type bat virus, but could rescue a modified bat-influenza virus that had the HA and NA coding regions replaced with those of A/PR/8/1934 (H1N1). This modified bat-influenza virus replicated efficiently in vitro and in mice, resulting in severe disease. Additional studies using a bat-influenza virus that had the HA and NA of A/swine/Texas/4199-2/1998 (H3N2) showed that the PR8 HA and NA contributed to the pathogenicity in mice. Unlike other influenza viruses, engineering truncations hypothesized to reduce interferon antagonism into the NS1 protein didn't attenuate bat-influenza. In contrast, substitution of a putative virulence mutation from the bat-influenza PB2 significantly attenuated the virus in mice and introduction of a putative virulence mutation increased its pathogenicity. Mini-genome replication studies and virus reassortment experiments demonstrated that bat-influenza has very limited genetic and protein compatibility with Type A or Type B influenza viruses, yet it readily reassorts with another divergent bat-influenza virus, suggesting that the bat-influenza lineage may represent a new Genus/Species within the Orthomyxoviridae family. Collectively, our data indicate that the bat-influenza viruses recently identified are authentic viruses that pose little, if any, pandemic threat to humans; however, they provide new insights into the evolution and basic biology of influenza viruses.

The identification of influenza virus-like sequences in two different bat species has generated great interest in understanding their biology, ability to mix with other influenza viruses, and their public health threat. Unfortunately, bat-influenza viruses couldn't be cultured from the samples containing the influenza-like nucleic acids. We used synthetic genomics strategies to create wild type bat-influenza, or bat-influenza modified by substituting the surface glycoproteins with those of model influenza A viruses. Although influenza virus-like particles were produced from both synthetic genomes, only the modified bat-influenza viruses could be cultured. The modified bat-influenza viruses replicated efficiently in vitro and an H1N1 modified version caused severe disease in mice. Collectively our data show: (1) the two bat-flu genomes identified in other studies are replication competent, suggesting that host cell specificity is the major limitation for propagation of bat-influenza, (2) bat-influenza NS1 antagonizes host interferon response more efficiently than that of a model influenza A virus, (3) bat-influenza has both genetic and protein incompatibility with influenza A or B viruses, and (4) that these bat-influenza lineages pose little pandemic threat.

Serological evidence of H7, H5 and H9 avian influenza virus co-infection among herons in a city park in Jiangxi, China

Extensive surveillance of influenza A viruses in different avian species is critical for understanding its transmission. Here, a breeding colony of Little Egrets and Black-crowned Night Herons was monitored both serologically and virologically in a city park of Jiangxi in 2009. A portion of herons had antibodies against H7 (52%), H5 (55%) and H9 (6%) subtype avian influenza virus (AIV) in egg yolk samples, and 45% had antibodies against different AIV serotypes (H5, H7 or H9) simultaneously. Greater numbers of samples with anti-AIV H5N1 recombination-4 (Re-4, clade 7) antibodies were measured compared with those containing anti-H5N1 Re-1 (clade 0) and Re-5 (clade 2.3.4) antibodies. Eight strains of H5 and 9 strains of H9 were isolated from poultry of nearby markets. These results indicate wild birds are at risk from infection and co-infection with H7, H5, and H9 subtypes. Investigation of wild bird infection might provide an early warning sign of potential novel AIVs circulating in the nearby poultry industry and even in human society.

A histidine residue of the influenza virus hemagglutinin controls the pH dependence of the conformational change mediating membrane fusion

The conformational change of the influenza virus hemagglutinin (HA) protein mediating the fusion between the virus envelope and the endosomal membrane was hypothesized to be induced by protonation of specific histidine residues since their pKas match the pHs of late endosomes (pK(a) of ∼ 6.0). However, such critical key histidine residues remain to be identified. We investigated the highly conserved His184 at the HA1-HA1 interface and His110 at the HA1-HA2 interface of highly pathogenic H5N1 HA as potential pH sensors. By replacing both histidines with different amino acids and analyzing the effect of these mutations on conformational change and fusion, we found that His184, but not His110, plays an essential role in the pH dependence of the conformational change of HA. Computational modeling of the protonated His184 revealed that His184 is central in a conserved interaction network possibly regulating the pH dependence of conformational change via its pKa. As the propensity of histidine to get protonated largely depends on its local environment, mutation of residues in the vicinity of histidine may affect its pK(a). The HA of highly pathogenic H5N1 viruses carries a Glu-to-Arg mutation at position 216 close to His184. By mutation of residue 216 in the highly pathogenic as well as the low pathogenic H5 HA, we observed a significant influence on the pH dependence of conformational change and fusion. These results are in support of a pK(a)-modulating effect of neighboring residues.

IMPORTANCE

The main pathogenic determinant of influenza viruses, the hemagglutinin (HA) protein, triggers a key step of the infection process: the fusion of the virus envelope with the endosomal membrane releasing the viral genome. Whereas essential aspects of the fusion-inducing mechanism of HA at low pH are well understood, the molecular trigger of the pH-dependent conformational change inducing fusion has been unclear. We provide evidence that His184 regulates the pH dependence of the HA conformational change via its pK(a). Mutations of neighboring residues which may affect the pK(a) of His184 could play an important role in virus adaptation to a specific host. We suggest that mutation of neighboring residue 216, which is present in all highly pathogenic phenotypes of H5N1 influenza virus strains, contributed to the adaptation of these viruses to the human host via its effect on the pKa of His184.

Influenza virus-glycan interactions

It has been known for many years that influenza viruses bind by their hemagglutinin surface glycoprotein to sialic acid (N-acetylneuraminic acid) on the surface of the host cell, and that avian viruses most commonly bind to sialic acid linked α2-3 to galactose while most human viruses bind to sialic acid in the α2-6 configuration. Over the past few years there has been a large increase in data on this binding due to technological advances in glycan binding assays, reverse genetic systems for influenza and in X-ray crystallography. The results show some surprising changes in binding specificity that do not appear to affect the ability of the virus to infect host cells.

Epitope mapping of the 2009 pandemic and the A/Brisbane/59/2007 seasonal (H1N1) influenza virus haemagglutinins using mAbs and escape mutants

mAbs constitute an important biological tool for influenza virus haemagglutinin (HA) epitope mapping through the generation of escape mutants, which could provide insights into immune evasion mechanisms and may benefit the future development of vaccines. Several influenza A (H1N1) pandemic 2009 (pdm09) HA escape mutants have been recently described. However, the HA antigenic sites of the previous seasonal A/Brisbane/59/2007 (H1N1) (Bris07) virus remain poorly documented. Here, we produced mAbs against pdm09 and Bris07 HA proteins expressed in human HEK293 cells. Escape mutants were generated using mAbs that exhibited HA inhibition and neutralizing activities. The resulting epitope mapping of the pdm09 HA protein revealed 11 escape mutations including three that were previously described (G172E, N173D and K256E) and eight novel ones (T89R, F128L, G157E, K180E, A212E, R269K, N311T and G478E). Among the six HA mutations that were part of predicted antigenic sites (Ca1, Ca2, Cb, Sa or Sb), three (G172E, N173D and K180E) were within the Sa site. Eight escape mutations (H54N, N55D, N55K, L60H, N203D, A231T, V314I and K464E) were obtained for Bris07 HA, and all but one (N203D, Sb site) were outside the predicted antigenic sites. Our results suggest that the Sa antigenic site is immunodominant in pdm09 HA, whereas the N203D mutation (Sb site), present in three different Bris07 escape mutants, appears as the immunodominant epitope in that strain. The fact that some mutations were not part of predicted antigenic sites reinforces the necessity of further characterizing the HA of additional H1N1 strains.

Origin of the European avian-like swine influenza viruses

The avian-like swine influenza viruses emerged in 1979 in Belgium and Germany. Thereafter, they spread through many European swine-producing countries, replaced the circulating classical swine H1N1 influenza viruses, and became endemic. Serological and subsequent molecular data indicated an avian source, but details remained obscure due to a lack of relevant avian influenza virus sequence data. Here, the origin of the European avian-like swine influenza viruses was analysed using a collection of 16 European swine H1N1 influenza viruses sampled in 1979-1981 in Germany, the Netherlands, Belgium, Italy and France, as well as several contemporaneous avian influenza viruses of various serotypes. The phylogenetic trees suggested a triple reassortant with a unique genotype constellation. Time-resolved maximum clade credibility trees indicated times to the most recent common ancestors of 34-46 years (before 2008) depending on the RNA segment and the method of tree inference.

Avian Influenza Virus with Hemagglutinin-Neuraminidase Combination H8N8, Isolated in Russia

We report the genome sequence of an avian influenza virus (AIV) subtype H8N8, isolated in Russia. The genome analysis shows that all genes belong to AIV Eurasian lineages. The PB2 gene was similar to a Mongolian low-pathogenic (LP) AIV H7N1 and a Chinese high-pathogenic (HP) AIV H5N2.

Structure of influenza virus N7: the last piece of the neuraminidase "jigsaw" puzzle

There are nine subtypes of influenza A virus neuraminidase (NA), N1 to N9. In addition, influenza B virus also contains NA, and there are two influenza virus NA-like molecules, N10 and N11, which were recently identified from bats. Crystal structures for all of these proteins have been solved, with the exception of N7, and there is no published report of N6, although a structure has been deposited in the Protein Data Bank. Here, we present the N7 and N6 structures at 2.1 Å and 1.8 Å, respectively. Structural comparison of all NA subtypes shows that both N7 and N6 highly resemble typical group 2 NA structures with some special characteristics, including an additional cavity adjacent to their active sites formed by novel 340-loop conformations. Comparative analysis also revealed new structural insights into the N-glycosylation, calcium binding, and second sialic acid binding site of influenza virus NA. This comprehensive study is critical for understanding the complexity of the most successful influenza drug target and for the structure-based design of novel influenza inhibitors.

IMPORTANCE

Influenza viruses impose a great burden on society, by the human-adapted seasonal types as well as by variants that occasionally jump from the avian reservoir to infect humans. The surface glycoprotein neuraminidase (NA) is essential for the propagation of the virus and currently the most successfully drug-targeted molecule. Therefore, the structural and functional analysis of NA is critical for the prevention and control of influenza infections. There are nine subtypes of influenza A virus NA (N1 to N9). In addition, influenza B virus also contains NA, and there are two influenza NA-like molecules, N10 and N11, which were recently identified in bats. Crystal structures for all of these proteins have been solved and reported with the exception of N7 and N6. The structural analysis of influenza virus N7 and N6 presented in this study therefore completes the puzzle and adds to a comprehensive understanding of influenza virus NA.

Swine and influenza: a challenge to one health research

The challenge of increasing swine production and a rising number of novel and known swine influenza viruses has prompted a considerable boost in research into how and why pigs have become such significant hosts for influenza viruses. The ecology of influenza A viruses is rather complicated, involving multiple host species and a segmented genome. Wild aquatic birds are the reservoir for the majority of influenza A viruses, but novel influenza viruses were recently identified in bats. Occasionally, influenza A viruses can be transmitted to mammals from avian species and this event could lead to the generation of human pandemic strains. Swine are thought to be "mixing vessels" because they are susceptible to infection with both avian and mammalian influenza viruses; and novel influenza viruses can be generated in pigs by reassortment. At present, it is difficult to predict which viruses might cause a human pandemic. Therefore, both human and veterinary research needs to give more attention to the potential cross-species transmission capacity of influenza A viruses.